Non of the free anti-virus or security software provides security or defends your computer against any malicious infection like Virus, trogan, Spyware, Mal-ware, etc.
All trial versions are just to show how real works, they only detects threats partially, free version does not repairs or quarantines.

Neither any valid version or purchased version guarantees you 100% security.
Because in 1hr approx 4000 new version or new types of viruses or infections are discovered.

But, security softwares works on Installing Updates basis.
you cant sit and update your ant-virus software, so that it can be aware about new
versions of infections, because than only it will protect your system.